This application relates generally to combustors and, more particularly, to gas turbine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Pollutant emissions from industrial gas turbines are subject to Environmental Protection Agency (EPA) standards that regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures that do not allow the fuel-air reaction to proceed to completion (HC & CO).
At least some known gas turbine combustors include between 10 and 30 mixers, which mix high velocity air with liquid fuels such as diesel fuel, and/or gaseous fuels such as natural gas. These mixers usually consist of a single fuel injector located at a center of a swirler for swirling the incoming air to enhance flame stabilization and mixing. Both the fuel injector and mixer are located on a combustor dome.
For most aeroderivative gas turbine engines, the fuel to air ratio in the mixer is rich. Since the overall combustor fuel-air ratio of gas turbine combustors is lean, additional air is added through discrete dilution holes prior to exiting the combustor. Poor mixing and hot spots can occur both at the dome, where the injected fuel must vaporize and mix prior to burning, and in the vicinity of the dilution holes, where air is added to the rich dome mixture. Other aeroderivative engines employ dry-low-emissions (DLE) combustors that create fuel-lean mixtures. Because the fuel-air mixture throughout the combustor is fuel-lean, DLE combustors typically do not have dilution holes.
One state-of-the-art lean dome combustor is referred to as a dual annular combustor (DAC) because it includes two radially stacked mixers on each fuel nozzle which appear as two annular rings when viewed from the front of a combustor. The additional row of mixers allows tuning for operation at different conditions. At idle, the outer mixer is fueled, which is designed to operate efficiently at idle conditions. At high power operation, both mixers are fueled with the majority of fuel and air supplied to the inner annulus, which is designed to operate most efficiently and with few emissions at high power operation. While the mixers have been tuned for optimal operation with each dome, the boundary between the domes quenches the CO reaction over a large region, which makes the CO emissions of these designs higher than similar rich dome single annular combustors (SACs). Such a combustor is a compromise between low power emissions and high power NOx.
Other known combustors operate as a lean dome combustor. Instead of separating the pilot and main stages in separate domes and creating a significant CO quench zone at the interface, the mixer incorporates concentric, but distinct pilot and main air streams within the device. However, the simultaneous control of low power CO/HC and smoke emissions is difficult with such designs because increasing the fuel/air mixing often results in high CO/HC emissions. The swirling main air naturally tends to entrain the pilot flame and quench it.